1. Technical Field
The present invention relates generally to internal combustion engines and, more particularly, to an automobile engine misfire detection system based on digitally filtered medium data rate crankshaft rpm sampling that exhibits misfire detection accuracy similar to systems having high data crankshaft rpm sampling, but with less associated computational complexity.
2. Discussion
Government regulations require automobile manufacturers to control the exhaust of engine combustion byproducts such as hydrocarbons, carbon monoxide, and nitrous oxide. Emission of such byproducts is typically controlled via a catalytic converter which operates at a high temperature and, through the use of a catalyst, burns the aforementioned unwanted exhaust byproducts to reduce automobile emissions. These catalytic converters enable automobile manufacturers to comply with government regulations in a cost-effective manner.
However, if an automobile engine misfires, an increased amount of unburned combustion byproducts is passed through the catalytic converter. Engine misfire occurs as a result of the absence of spark in a cylinder, poor fuel metering, poor compression, or other similar conditions. Over time, regular engine misfire can lead to damage of the catalyst in the catalytic converter and, consequently, increased amounts of unburned byproducts being admitted into the atmosphere.
As a result, regulatory agencies such as the California Air Resources Board (CARB) require that many motor vehicles with feedback fuel control systems be equipped with an emission malfunction indicator that identifies a misfiring engine and the particular malfunctioning component or components. Thus, upon the malfunction indicator being activated, the vehicle operator could proceed to a qualified vehicle repair center to have the malfunctioning component repaired or replaced before an excessive amount of exhaust byproducts is emitted into the air by the vehicle.
Typically, these malfunction indicators generate data allowing identification of specific misfiring engine cylinders. In particular, the CARB rules, known as On-Board Diagnostics II guidelines, mandate that the automobile manufacturer specify a percentage of misfires out of the total number of firing events necessary for determining malfunction for: (1) the percent misfire evaluated in a fixed number of revolution increments for each engine speed and load condition which would result in catalyst damage; (2) the percent misfire evaluated in a certain number of revolution increments which would cause a motor vehicle to fail a federal test procedure by more than 1.5 times the CARB standard if the degree of misfire were present from the beginning of the test; and (3) the degree of misfire evaluated in a certain number of revolution increments which would cause a motor vehicle to fail inspection and a maintenance program tailpipe exhaust emission test. It is contemplated that similar rules are or may be enacted by other states as by the federal government in the foreseeable future.
Government regulations such as those mandated by CARB also require that automobile manufacturers be able to provide information identifying misfiring engine cylinders. This misfire information is typically collected and stored in a computer memory associated with the automobile engine and later downloaded at a service center and is used in diagnostic testing of the vehicle. One misfire detection and identification approach is disclosed in U.S. Pat. No. 5,361,629, issued Nov. 8, 1994 entitled "Single Sensor Misfire Detection Apparatus and Method for an Internal Combustion Engine". The misfire detection approach disclosed in the aforementioned patent senses crankshaft rotation and calculates a crankshaft velocity based on the sensed rotation of a sensor wheel associated with the crankshaft. The calculated crankshaft velocity changes or a compensated velocity change is compared to a predetermined stored crankshaft velocity range to determine engine misfire.
Additional engine misfire detection approaches are disclosed in U.S. Pat. No. 5,574,217 issued Nov. 12, 1996 for "Engine Misfire Detection with Compensation for Normal Acceleration of Crankshaft"; U.S. Pat. No. 5,544,521 issued Aug. 13, 1996 for "Engine Misfire Detection with Rough Road Inhibit"; and U.S. Pat. No. 5,602,331 issued Feb. 11, 1997 for "Engine Misfire Detection with Cascade Filter Configuration". The approaches disclosed in the above mentioned patents and patent applications relate to engine misfire detection including sorting of a plurality of changes in crankshaft angular velocity, as measured by sensing rotation of a sensor wheel associated with the crankshaft, over a predetermined series of cylinder firings in averaging the two middle most angular velocity changes to provide an average change in velocity value. The deviation is determined between the change in angular velocity for a selected cylinder and the average change in velocity value. Misfires are detected as a function of a comparison of the deviation with a predetermined threshold velocity value stored in a system controller.
Generally, a number of conventional misfire detection approaches work well at engine speeds below 4000 rpm. At these lower engine speeds, with cylinder identification engine misfire detection can be realized through monitoring of engine rpm alone. Even at low engine speeds, however, transient signals caused by conditions such as powertrain bobble, engine noise, changing of gears, or engine acceleration and deceleration may cause false engine misfire detection. Additionally, at higher engine speeds of typically greater than 4,000 rpm, engine induced crankshaft flex, or torsional vibration, can cause false engine misfire detection.
To reduce the number of false engine misfire detections, the approach disclosed in pending U.S. application Ser. No. 08/511,125 filed Aug. 4, 1995 for "Engine Misfire Detection with Digital Filtering" was developed to filter out noise related signals at both low and high engine frequencies to achieve an enhanced signal to noise ratio for detecting engine cylinder misfires. A crankshaft velocity signal is generated by measuring the amount of time expired over a 40.degree. crankshaft angular displacement interval for a cylinder expansion stroke. A digital high pass finite impulse response (FIR) filter operates at low engine speeds to filter out low frequency engine noise. At high engine speeds, a low pass FIR filter operates to filter out high frequency engine noise from the crankshaft rotation signal.
In addition, many conventional misfire detection approaches analyze engine firing frequency through a fast fourier transform (FFT) or other similar frequency domain-based analysis to determine whether an engine misfire has occurred for a particular cylinder firing event. However, the aforementioned approaches are typically complex and do not take into account manifold absolute pressure (MAP) in setting a particular threshold value for engine misfire detection.
Further, many conventional misfire detection methods utilize low data rate sampling of engine crankshaft velocity or acceleration where crankshaft angular velocity is sampled only once per cylinder firing event. With low data rate sampling, higher multiples of the engine firing frequency, which often contain valuable misfire information for higher engine speeds, frequently are folded back, or aliased, into lower noise-related engine frequencies. These aliased signals are thus either filtered out completely from the misfire detection signal or cause misinterpretation of cylinder firing event data.
Further, pending U.S. patent application Ser. No. 08/691,100 filed Aug. 1, 1996 for "Real Time Misfire Detection for Automobile Engines" and pending U.S. patent application Ser. No. 08/695,766 filed Aug. 1, 1996 for "Spatial Frequency Implemented Digital Filters for Engine Misfire Detection" relate to a misfire detection technique that can identify multiple engine misfires through evaluation of engine speed and average MAP data alone, without the need for complex frequency-based methods of analysis, such as Fast Fourier transform-based methods, or the need for additional engine system data such as engine temperature or throttle feedback data. Also, the above pending applications relate to an engine misfire detection method in which one of a multiple of FIR filter banks is operative over a predetermined range of engine speeds to provide effective removal of noise related signals from the signal representing crankshaft angular velocity that samples crankshaft angular velocity at a high data rate of 18 samples per cylinder firing event so that change in crankshaft angular velocity over a given band of engine frequencies around the engine firing frequency may be determined to produce more accurate engine misfire calculations and that utilizes MAP data to set a particular signal threshold for a given set sampled crankshaft angular velocities.
Also, pending U.S. patent application Ser. No. 08/719,304 filed Sep. 16, 1996 for "Deconvolution Method of Resonance Detection and Removal from Crankshaft Speed Measurements" and pending U.S. patent application Ser. No. 08/755,046 filed Nov. 22, 1996 for "Mixed Sampling Rate Processing for Misfire Detection" disclose systems and techniques for improving misfire detection accuracy.
While the above described misfire detection approaches are effective in detecting engine misfire, there is still room for improvement in the art. In particular, there is a need for a misfire detection system that utilizes medium data rate crankshaft rpm data sampling through measurement of the angular velocity of a crankshaft sensor wheel at a medium data rate at or near the Nyquist rate to achieve misfire detection with an accuracy typically associated with systems utilizing high data rate crankshaft sampling, and with less computational complexity. There is also a need for a misfire detection technique that utilizes signal processing and statistical decision techniques in addition to medium data rate sampling, to further enhance system accuracy.